1. Field of the Invention
The present invention is related generally to optical imaging, and more particularly it is related to a catadioptric optical system with a multi-reflection optical element for high numerical aperture imaging. The catadioptric optical system may find industrial application in microscope objective systems, lithographic projection systems or telescopic systems, among others.
2. Description of the Related Art
Imaging apparatuses, such as microscope objectives, lithographic projection systems, or even telescopes, use purely reflective (catoptric), purely refractive (dioptric), or a combination of reflective and refractive (catadioptric) optical elements to form an image of an object. Conventional optical systems used in these instruments often lack compactness due to the number of optical elements necessary to correct aberrations and produce good image quality.
Four main design approaches have been adopted in the design of conventional optical systems, as follows, Total Internal Reflection (TIR), dioptric, catoptric, and catadioptric. In the TIR approach, obscuration is the main limiting factor which cannot be reduced beyond a certain limit. All refractive (dioptric) optical systems are the default standard for optical systems in the visible wavelength ranges with low numerical aperture (NA). However, as NA increases, correction of chromatic aberrations requires an ever increasing number of optical elements, which results in the total length (dimension in the direction of incident light) of the optical system being relatively larger than in the other approaches. The catoptric approach, which uses all reflective elements, is characterized by negligible chromatic aberrations controlled by appropriately designed mirror coatings, but central obscuration and Petzval curvature are relatively large. The catadioptric approach is a compromise between the all reflective (catoptric) and all reflective (dioptric) approaches. Although a catadioptric optical system allows for better control of aberrations and obscuration ratio, chromatic aberrations induced by individual lenses and Petzval curvature induced by multiple mirrors are appreciable and contribute significantly to quality degradation in the final image.
Certain catadioptric designs have managed to balance certain aberrations and scale down the overall length of the optical system by reducing the number of optical elements included therein. For example, U.S. Pat. No. 5,650,877, international publication number WO2008/101676 (herein “WO2008/101676”), FIG. 8 of U.S. Pat. No. 7,646,533, and the article “A New Series of Microscope Objective: I. Catadioptric Newtonian Systems,” JOSA 39, No 9, 719-723 (1949), by Grey et al. have a common feature in that the first optical element is a solid lens in which light is reflected two times within it. In this manner, the extra space and aberrations of at least one reflective element and one refractive element is avoided. However, high obscuration ratios and limited numerical aperture are pervasive in these arrangements.
In addition, it has been proposed that the first optical element can be made to reflect light more than two times within it. For example, U.S. Pat. No. 7,898,749 to Ford et al., (herein “Ford”) discloses a multiple reflective lens having multiple, substantially-annular and concentric, reflective zones of appropriate directionality, where the zones are illuminated in sequence by a light flux incident on the outer edge of the lens. According to Ford, the above-mentioned lens can be fabricated in a variety of manners, but preferably by way of diamond machining. Two surfaces may be formed as the two sides of a single solid element (a solid lens), or two lenses having the multiple reflective zones are designed as two mechanically separate elements on each side of an air gap (a hollow lens). Depending upon the design of the lens, focusing with the multiple reflective lens can be accomplished by moving the lens relative to the image plane, as in conventional lenses, or by adjusting the gap between the two reflective surfaces.
One of the problems encountered with the multiple reflection lens proposed by Ford is that fabrication becomes excessively costly and complicated. For example, since the substantially-annular and concentric reflective zones must have appropriate directionality so that the zones are illuminated in sequence by incident light rays, fabrication precision must be very high. In addition, since the concentric reflective zones must have appropriate directionality so that the zones are illuminated in sequence by incident light, the angle of incident light must be restricted to only those angles that match the appropriate directionality of the reflective zones. Moreover, when incident light with steep incident angles is used, a large obscuration ratio and limited field of view may be observed. In other words, the lens with multiple concentric zones having predetermined directionality would lead to an optical lens design with tight fabrication tolerances, large obscuration ratios, limited field of view, and low numerical aperture.
David Shafer (herein “Shafer”), in an article entitled “Wide-Angle Flat-Image Unobscured Telescope with Four Spherical Mirror”, Journal of the Optical Society of America, Vol. 73, No. 12, published 1983, describes that multiple reflections (more than two) may be useful to correct for spherical aberration, coma and astigmatism, but it substantially more obscuration than a two-reflection design unless off-axis illumination is adopted. The design proposed by Ford is similar to that of Shafer in that light is incident off-axis and in that multiple reflective zones not forming a single surface are used to achieve four reflections.
Accordingly, there is a need for optical systems that can provide minimum obscuration, correction of chromatic aberration and Petzval curvature, and allow for appropriate alignment without undue difficulty. That is, it is particularly desirable an easy-to-fabricate multi reflection optical element that can provide appropriate correction of aberration, small obscuration ratio, large field of view, and small RMS wavefront errors in a broad spectral band.